1. Field of the Invention
The present invention relates to an organic EL (Electroluminescence) element having a structure in which an organic EL layer is put between a pair of electrodes, an organic EL display device employing the element, organic EL material, and a plane emission device and a display device employing the material.
2. Description of the Prior Art
In recent years, the organic EL element (the organic LED) is well appreciated as the spontaneous emission element. The organic EL element has the advantage that such element can be driven by the low voltage rather than the inorganic EL element and also the advantage that such element can be manufactured without complicated manufacturing processes in contrast to the inorganic LED in the prior art.
Also, in comparison with the liquid crystal display device that is widely used at present as the display of the mobile device, the organic EL element has a quick response speed, has a simple device structure, and needs no back-light. Therefore, the organic EL element has the advantage that such element can be reduced in weight. In addition, since the organic EL element is the solid state element, such element has the merit that it is resistant to the impact.
The organic EL element has the structure that the EL emission layer (the organic EL layer) is put between the cathode and the anode. The metal having the large work function is employed as the anode and the metal having the small work function is employed as the cathode, so that supplies of the hole and the electron can be made smooth. Normally, ITO (Indium-Tin Oxide) as the transparent conductor is employed as the anode.
Also, the metal containing the alkaline metal or the alkaline earth metal such as Na (sodium), Na—K (sodium-potassium) alloy, Mg (magnesium), Li (lithium), Mg/Cu (magnesium/copper) mixture, In (indium), etc. is employed as the cathode.
Alq3, BeBq3, DCM, DPVBi, quinacridone derivative, coumalin, etc., for example, is employed as the EL emission layer. Normally, the monomer EL emission layer is formed by the vacuum evaporation method, and the polymer EL emission layer is formed by the spin coating method. Accordingly, the polymer EL emission layer has the advantage that the film formation is easy and also the mechanical strength is high.
In addition, in order to lower the operating threshold voltage of the organic EL element, the trial for forming the buffer layer on the cathode side or the anode side is made. For example, sometimes the layer made of ruthenium oxide (referred to as “RuO” hereinafter), molybdenum oxide (referred to as “MoO” hereinafter), or vanadium oxide (referred to as “VO” hereinafter) is formed as the buffer layer between the anode and the organic EL layer in the prior art. The layer made of RuO, MoO, or VO is formed by the sputter method.
By the way, if the organic EL element is used for a long time, the luminous efficiency is lowered due to the influence of the oxygen and the moisture and also the defect called the dark spot is caused. This is due to the fact that the alkaline metal or the alkaline earth metal employed as the cathode is easily oxidized.
In Patent Application Publication (KOKAI) Hei 7-169567 as the preceding literature, in order to avoid the degradation of the luminous efficiency of the organic EL element due to the oxygen and the moisture, it is proposed that the laminated body consisting of the anode, the organic EL layer, and the cathode should be covered with the layer formed of material that can adsorb, occlude, or consume the oxygen (referred to as a “sealing layer” hereinafter).
Then, the material such as magnesium oxide, magnesium carbonate, iron oxide, titanium oxide, bentonite, or the like, that is impregnated with platinum, palladium, rhodium, ruthenium, or silver at a concentration of less than 5 wt %, etc. are set forth as the material of the sealing layer.
However, the problems discussed in the following are pointed out in the organic EL element disclosed in the above literature. That is, since the laminated body consisting of the anode, the organic EL layer, and the cathode is covered with the insulating sealing layer, the leading electrode that is extended to the outside of the sealing layer must be provided to the anode and the cathode respectively. As a result, the manufacturing steps become complicated and also the rise of the product cost is brought about.
Also, as described above, sometimes the buffer layer made of RuO, MoO, or VO is formed by the sputter method in order to lower the operating threshold voltage.
However, this method has the drawback that the large unevenness is formed. If it is tried to form the RuO2 layer of 30 nm thickness by the sputter method, for example, the hillock having a height of 50 to 100 nm is locally generated. As a result, there is the possibility that, if the thickness of the organic EL layer is reduced, conversely the short-circuit defect is caused.
In addition, in case the full-color image display device using the organic EL elements is fabricated, the organic EL elements for the red color emission, the organic EL elements for the green color emission, and the organic EL elements for the blue color emission must be aligned in both the horizontal direction and the vertical direction in the predetermined order.
Therefore, the technology for patterning finely the organic EL layers is requested. As the fine pattern technology used in manufacturing the semiconductor device, the lift-off method and the etching method are well known.
However, in the case of the monochromatic image display device, it may be considered that, for example, the upper electrodes (the cathodes) of respective pixels are formed by the lift-off method. But it is difficult to apply the lift-off method to the manufacture of the full-color image display device.
Further, since the organic EL layer is formed of the monomer or polymer organic, such organic EL layer has the drawback that it cannot be worked by the fine pattern technology such as the dry etching, etc.
Moreover, the above organic EL element is noted with interest as the back-light device in place of the cold-cathode lamp that is employed in the liquid crystal display device. The cold-cathode lamp, that is employed as the back-light device in the prior art, has the low luminous efficiency and also occupies about 30% of the overall cost of the liquid crystal display device, which is a factor of the cost up.
The monomer and polymer materials are known as the organic EL material. Normally the film of the monomer EL material can be formed by the vacuum evaporation method, whereas the film of the polymer EL material can be formed by the coating method. Therefore, the polymer EL material is advantageous in manufacturing cost.
Since the back-light device using the polymer organic EL material can be driven by the low voltage, the reduction in the consumption power of the liquid crystal display device can be expected and also the cost down and the reduction in size of the device can be expected rather than the cold- cathode lamp in the prior art.
However, the actual circumstances are that the organic EL material that is available in practical use has not been developed yet.